Efficiency of operation is of paramount importance in gas turbines, in particular aero gas turbines.
One factor reducing the efficiency of gas turbines is the thermal expansion and contraction of different turbine machinery parts associated with alterations of power. A particular issue concerns the clearances between the tips of the turbine blades and a casing which is mounted radially outwardly of and circumferentially surrounds the blade tips. For example, at the exhaust end of the gas turbine, the turbine machinery has the problem of matching the casing diameter to the tip diameter of the blades. When the turbine machinery power is altered, the casing changes diameter relatively quickly in response to the turbine gas temperature change, but a disk supporting the blades reacts more slowly. This leads to excessive tip/casing clearances, lowering the efficiency of the turbine, and therefore to higher fuel consumption. It is well known in the industry that small turbine blade tip clearances provide lower operational specific fuel consumption and hence allow large fuel savings. As a counter-measure, turbine case cooling is applied, in which cold air is blown onto the casing to control its diameter.
U.S. Pat. No. 5,205,115 discloses a system for cooling a turbine casing, wherein circumferential air suppliers located on a casing of the turbine are used to provide cooling air for the turbine casing. The air suppliers are circumferential tubes with a number of holes (so-called piccolo tubes) through which the cooling air escapes onto thermal control rings, disposed circumferentially on the casing. The thermal control rings may move a shroud radially inward or outward and thereby adjust the blade tip clearance upon thermal contraction or expansion. The cooling air is provided from the fan flow of the turbine. It is compressed from the fan and would normally be accelerated through the final nozzle of the turbine to provide thrust. Since a certain amount of cooling air is bled from the high pressure section of the turbine, a reduction of thrust or efficiency occurs.
In this setup, tight control of the geometry is necessary. In particular, the distance between the holes in the air ducts and the thermal control rings is important to provide efficient cooling. Additionally, the cooling needs to be adjusted such that no circumferential cooling gaps occur, which would lead to a distortion of the casing shape, and therefore increased wear and tear, caused by temperature differences. A further drawback results from leakages in the cooling air suppliers, which reduce the efficiency of the cooling and increase the amount of cooling air which needs to be bled from the turbine machinery, therefore reducing thrust or efficiency. Overall, the system is expensive, complicated and heavy and is prone to mechanical malfunction.